I honestly think that this is a great idea. It's a novel approach to extending a kernel, especially considering NetBSD doesn't have that big of a market share and expressly focusses on supporting as many platforms as possible. Prototyping new features or certain drivers in a portable scripting language may give them a leg up in the functionality aspect of the race.
Also, LUA is super fast, small, easy to learn, concise and the C API/embedding it is straight forward.
You may knock it all you want, "because kernel land is C land", but after all bias being said and flamed, this may turn out to be a really fun, interesting and possibly very useful idea.